Axial-flow fans of this type are known from DE-A 33 04 296. For the purpose of improving the semi-axial flow in this type of fan, an "annular disk" was attached to the downstream region of the cylindrical hub and resulted in flow stabilization by virtue of an annular vortex being formed. In addition to this annular disk, a front ring was provided, as a flow-directing surface, in the region of the fluid-friction clutch. The front ring deflected the radial clutch flow in the direction of the annular disk. The disadvantage with this known design was that the flow of this fan, in particular in the hub regions, was not satisfactory for all operating states.
The problems of an axial-flow fan for radiators of motor vehicles are described in detail in Behr's company publication "Dusen-Mantellufter fur Nutzfahrzeug-Kuhlanlagen" (Injector-bushing fan for cooling systems of commercial vehicles), by Kurt Hauser, published in MTZ Motortechnische Zeitsschrift, 53rd year, issue 11/92. Point 3 of the publication discusses the throttle coefficient and the different operating ranges of the axial-flow fan. It can be gathered from this description that the axial-flow fan, which is installed in the motor vehicle between the radiator and the internal combustion engine, and is thus subjected to relatively pronounced throttling, has flow passing through it semi-axially in most cases. There is also a superposition of the radially directed flow of the clutch, arranged in the interior of the fan, upon the semi-axial flow. These differing and changing boundary conditions make it difficult to design such a fan.
DE-A 29 02 135 discloses a fan drive for a radiator of an internal combustion engine, wherein an axial-flow fan is driven via a fluid-friction clutch. The axial-flow fan is fastened to the clutch via a hub cross, through which flow takes place in the axial direction. The fan hub is shortened in the axial direction with respect to the blade depth, resulting in the leading blade edge projecting slightly in front of the hub end side. An annular gap is left between the hub and the fluid-friction clutch and, through this annular gap, the fan takes in a secondary airstream in the forward direction from the rear side of the fan in order to cool the rear side of the clutch. This annular gap increases the overall axial depth of the fan and the external diameter of the fan, which is undesirable in present motor vehicles. Furthermore, the efficiency of this fan is impaired by the secondary airstream.
The problems identified above are not intended to be exhaustive but rather are among many which tend to reduce the desirability of previous axial-flow fans. Other problems may also exist. However, those presented above should be sufficient to demonstrate that currently known solutions are amenable to worthwhile improvement.